Iron-sulfur reaction center

Figure 1. Pheophytin-quinone and iron-sulfiir Reaction Centers. The dotted line represents the absorption of light by theprimary electron donor (Chl2 or BChl2).Thelineshows the energy transfers in the Reaction Center, from the PSII tyrosine residue (Yz), through the monomer bacttriochlorophyll (BChl), A) the monomer bacterio-pheophytin (BPhe), or B) pheophytin (Phe) and quinone transfer components, QA and QB, in the pheophytin-quinone type of Reaction Center, and Q through the monomer chlorophyll (Chi), quinone (Q) and F components in the iron-sulfur Reaction Centers.

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

IRON-SULFUR CENTERS INVOLVED IN PHOTOSYNTHETIC LIGHT REACTIONS... 

IRON-SULFUR CENTERS IN PHOTOSYNTHETIC LIGHT REACTIONS 341... 

Cluster Fx was also identified via its EPR spectral features in the RCI photosystem from green sulfur bacteria 31, 32) and the cluster binding motif was subsequently found in the gene sequence 34 ) of the (single) subunit of the homodimeric reaction center core (for a review, see 54, 55)). Whereas the same sequence motif is present in the RCI from heliobacteria (50), no EPR evidence for the presence of an iron-sulfur cluster related to Fx has been obtained. There are, however, indications from time-resolved optical spectroscopy for the involvement of an Fx-type center in electron transfer through the heliobacterial RC 56). 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

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